White light emitting diodes have been recently and extensively investigated by combining a blue light emitting diode element and a blue-light absorbing/yellow-light emitting phosphor. Examples of these have been proposed and presented in various literature including patent-related references (see patent-unrelated reference 1, and patent-related references 1 through 5, for example). Recently, lighting instruments, lighting equipments, and lighting apparatuses based on the aforementioned combination have been put into practical use.
An example of a phosphor
frequently used in the above applications, is a yttrium/aluminum/garnet based phosphor activated by cerium, represented by a general formula (Y, Gd)3(Al, Ga)5O12:Ce3+. However, the white light emitting diode comprising a blue light emitting diode element and a yttrium/aluminum/garnet based phosphor tends to emit bluish white light due to lack of a red component, thereby problematically exhibiting deviation in a color rendering property.
Meanwhile, lighting techniques have been diversified in usages, usage schemes, and needs, thereby demanding diversified color tone designs including realization of not only white color having a higher color temperature in the aforementioned lighting techniques utilizing light emitting diodes, but also white colors having various color temperatures such as seen in conventional ordinary lighting instruments, respectively. For example, there has been a need for a white light emitting diode for achieving a white color which provides warmness and is referred to as an “incandescent color”. Under such circumstances, there has been research into a white light emitting diode including two kinds of mixed and dispersed phosphors, such that a red component lacking in case of a yttrium/aluminum/garnet based phosphor, is compensated for by an additional red phosphor.
Such a white light emitting diode has been proposed in a patent-related reference 4 (“white light emitting element”), a patent-related reference 5 (“nitride phosphor and production method thereof”), and the like. However, the above-described problem has not been fully solved even by such research, which deviates from a situation where the needs for diversified color schemes and chromaticities are fully dealt with, while such proposals are also insufficient in emission intensity, thereby still exhibiting a problem to be solved. Namely, the arrangement described in the patent-related reference 4 exhibits a problem in that a red phosphor used therein contains Cd, i.e., cadmium. However, it has been recently proposed to eliminate cadmium and cadmium-including compounds from usage, based on environmental pollution concerns, and to alternatively use substances free of cadmium. It is thus desirable to design of fluorescent materials in view of the above consideration.
Further, red-light emitting phosphors described in a patent-related reference 5 and exemplarily represented by Ca1.97Si5N8:Eu0.03, are not problematic in that the phosphors include no elements like cadmium which are a concern with respect to environmental pollution, but are insufficient in emission intensity, so that further improvement is still desired. Moreover, the technical concepts described in the patent-related references 4 and 5 have merely and exclusively referred to realization of white color. Lighting techniques are diversified in usage as mentioned above, and decorative effects are also demanded. Thus, diversified color tones and tints are demanded, thereby in turn demanding various light sources for preparing and realizing color tones therefor. Namely, such needs have not been satisfied by white light emitting diodes only, so that there has been the desired realization of light emitting diodes of various intermediate colors. Additionally, it has been also desired to extend a chromaticity range as wide as possible from a desired color tone so as to attain a sufficient color tone expression.    [Patent-unrelated reference 1] M. Yamada et al, Jpn. J. Appl. Phys., vol. 42 (2003), pp. L20-23    [Patent-related reference 1] JP-2900928    [Patent-related reference 2] JP-2927279    [Patent-related reference 3] JP-3364229    [Patent-related reference 4] JP-A-10-163535    [Patent-related reference 5] JP-A-2003-321675